Adds a boolean property to RTCConfiguration.offerExtmapAllowMixed() to enable the extmap-allow-mixed attribute in a session description protocol (SDP) offer. The SDP attribute extmap-allow-mixed, as defined in RFC8285, will be included in the SDP offer if this property is set to true. The SDP attribute extmap-allow-mixed is supported from Chrome 71, but due to backwards compatibility problems it was not included in the SDP offer by default.
Docs: https://docs.google.com/document/d/1PY2syYX5w9PYwGOCJchNs366NsWpuzVWIQYKscblYco/